This invention relates generally to semiconductors and more particularly to packaging of semiconductors.
As is known in the art, semiconductors, such as a mesa-shaped IMPATT diode, are often used in applications where high output power at microwave frequencies is required. When used in such applications, the IMPATT diode, for example, is generally bonded to a heat spreader member to dissipate or extract from the diode the heat generated by the diode. Due to the temperature at which the diode operates, it is important to provide a uniform bond between a bottom contact portion of the diode and the heat spreader. Such a uniform bond is required to minimize the thermal resistance of the mounted diode thereby permitting the diode to operate at high output power levels without damaging the diode. However, since such diodes are very small and fragile providing uniform bonds is very difficult.
A technique for bonding the IMPATT diode generally involves placing the bottom contact of the diode on the heat spreader and bringing a bonding tool into contact with an upper electrode contact of the diode while the heat spreader is raised to an elevated temperature. Pressure is applied to the mesa by the bonding tool and in combination with the elevated temperature of the heat spreader a thermocompression bond is provided between the bottom contact and the heat spreader. If the bonding tool is placed directly over the mesa portion of the diode and unguided during bonding, there is a tendency for the tool to tilt the diode since there is a slight tendency for the bonding operator to apply the bonding tool at an angle off of perpendicular with respect to the heat spreader and lower contact of the diode. Thus, the resulting bond provided between the diode and the heat spreader will not be uniform. The nonuniform bond will result in a reduction in the area of the diode bonded to the heat spreader and will cause a concomitant increase in the thermal resistance of the diode. Thus, in order to provide a uniform bond between the mesa and the heat spreader, the bonding tool must be brought to bear upon the upper contact substantially perpendicularly to the lower contact/heat spreader interface.
One technique used during the bonding operation is to guide the bonding tool perpendicularly to the diode contact/heat spreader interface while supporting the diode package during bonding with a "tip and tilt" device which permits adjustment of the position of the diode package. This device includes a pair of concentric overlaying steel plates joined at their center by a flexible steel pin. The plates are separated by a predetermined distance. Two screws located 90 degrees apart are tapped into the periphery of the upper plates and loaded against the lower plate by two corresponding opposing springs. The springs are in compression between the plates. This device permits universal angular orientation of one plate against the other. The bonding tool is disposed at an end of a pivotally mounted arm. The plates are adjusted such that the bonding tool will contact the diode at an angle normal to the diode contact/heat spreader interface. While this technique provides some improvement in the uniformity of bonds between the mesa and the heat spreader, imperfections in the dimensions of the package generally require that each individual package be re-aligned by a reiterative adjustment of the tip and tilt device prior to performing the bonding operation. This is a very long and tedious process.